Analysis of volatile organic compounds emitted from aircraft carpets: comparison using headspace and dynamic chamber tests

Transcription

1 Journal of Chongqing University (English Edition) [ISSN ] Vol. 13 No. 1 March 2014 doi: /j.issn To cite this article: WANG Chao, YANG Xu-Dong, GAO Peng. Analysis of volatile organic compounds emitted from aircraft carpets: comparison using headspace and dynamic chamber tests [J]. J Chongqing Univ: Eng Ed [ISSN ], 2014, 13(1): Analysis of volatile organic compounds emitted from aircraft carpets: comparison using headspace and dynamic chamber tests WANG Chao, YANG Xu-Dong, GAO Peng Department of Building Science, Tsinghua University, Beijing , P. R. China Received 24 January 2014; received in revised form 18 March 2014 Abstract: Volatile organic compounds (VOCs) emitted from three types of carpets used in aircrafts were compared by using headspace and dynamic chamber tests. The headspace samples contained many compounds that were not detected in the dynamic chamber test; in addition, the dominant VOCs found by these two methods were different. The findings indicate that for highly sorptive materials such as carpets, headspace analysis may give inaccurate indication of actual VOC emissions, and it is necessary to conduct dynamic chamber tests over a certain period of time in order to identify the true emission characteristics. From the dynamic chamber tests, 2-ethyl-1-hexanol was the main VOC emitted from all three carpets. The study also examined the emission characteristics of aircraft carpets. In all experiments, total VOC (TVOC) concentration peaked within a few hours after the start of the experiment and was followed by rapid decay. The emission parameters of TVOC emitted by all three carpets were calculated and the simulated data matched the measured data well. Keywords: aircraft cabin carpets; volatile organic compounds; headspace test; dynamic chamber test CLC number: X511 Document code: A 1 Introduction a Volatile organic compounds (VOCs) are common air pollutants in the indoor environment. They are used widely in many household products, such as paints, WANG Chao ( 王超 ): carsal-007@163.com. Corresponding author, YANG Xu-dong ( 杨旭东 ): xyang@mail.tsinghua.edu.cn; Tel: ; Fax: Funded by the National Basic Research Program of China (973 Program) under Grant No. 2012CB flooring materials, and furniture. These compounds can be harmful to human health, and are associated with eye, nose and throat irritation, headaches, loss of coordination, nausea, damage to the liver, kidneys, and central nervous system, etc. [1] Carpet products cover extensive areas and include complex components, and so have long been a topic of research interest in the literature on VOC emissions. The many experimental studies of VOC emissions from carpets generally have two purposes: to determine the VOC emission pattern and parameters; and the factors influencing VOC emissions, such as temperature, air velocity, relative humidity, etc. 1

2 Gunnarsen [2] investigated the influence of area-specific ventilation rate on VOC emissions from carpet. Sample carpets were placed in a chamber in which ventilation, temperature, and relative humidity (RH) were controlled, and the VOC concentrations under different ventilation rates were compared. A study by Van der Wal et al. [3] investigated the influence of temperature on emissions of VOCs from many building products including carpets. Wolkoff [4] examined the influence of temperature, humidity, and air exchange rate on the long-term (250 d) emission of VOCs from carpets in a dynamic test chamber. All of the aforementioned studies used a dynamic test chamber in which method the material was put into a dynamic chamber with given temperature, RH and ventilation, then the VOCs emitted were measured at a specified time interval. However, the characteristics of VOC emission from carpets can also be examined by using the headspace method [5], in which the carpets were sealed in a bag and VOC emitted were measured after a specified time. It is widely known that these two methods produce different results, but no previous study has examined this issue in detail. This could lead to inadequate understanding of experiment results. Carpets are the main type of floor covering used in aircraft cabins. Aircraft cabins are a small, confined space. Although the total air supply volume according to ASHRAE was 33.9 m 3 h 1 for each person [6], the air supplied to each passenger may be 1 m 3 to 2 m 3 due to the high occupant density [7]. As a result, pollutants emitted by products used in the cabin can be much more harmful to the health of passengers and crew members than in standard buildings. However, few studies have been conducted on emissions from aircraft carpets [7]. In this study, three types of aircraft carpets were tested using both headspace test and dynamic chamber test. The objectives of this study were: (a) to compare the results obtained via headspace test and dynamic chamber test, and (b) to characterize VOC emissions from aircraft carpets using dynamic chamber test. 2 Materials and methods 2.1 Carpets The carpets used in this study were wool, synthetic, and mixed-type. All carpets were new and manufactured for airplane use. They were supplied directly from the Boeing Company and were labeled and kept in Tedlar bags prior to testing within one month. All carpets were tested for VOC emissions by using both the headspace test and dynamic chamber test. 2.2 Headspace test The purpose of the headspace test is to provide primary information about the types of compounds emitted, and to identify the dominant compounds emitted from the material. This method is widely used for qualitative analysis because it is easily undertaken and requires only simple instruments. In the first step of the headspace test, the carpet sample was cut into the desired size of about 15 cm 14 cm and flushed with clean air. The sample was then placed into a Tedlar bag (approximately 30 cm 30 cm). The air in the bag was alternately exhausted and refilled with ultra-zero grade compressed air (repeated three times) through the stainless steel sampling valve of the bag. The carpet sample was left in the bag for 48 h (the total air volume of the bag was nearly 15 L and the temperature was controlled at 23 C ±1 C), and then an air sample of 10 L was collected via Tenax adsorption tube at a sampling rate of 1 L/min. 2.3 Dynamic chamber test Dynamic chamber tests were conducted to determine 2 J. Chongqing Univ. Eng. Ed. [ISSN ], 2014, 13(1): 1-10

3 the emission of VOCs over time. The chamber had a capacity of 53 L and was constructed as per ASTM D5116 [8]. To avoid sink effects on interior surfaces, the material used to construct the chamber was nonadsorbent, chemically inert, and with a smooth surface. A schematic diagram of the dynamic measurement system is shown in Fig. 1. The chamber was cleaned with methanol and flushed with deionized water before each measurement, and then a chamber blank was analyzed to ensure the TVOC concentration was lower than 5 µg/m 3. The size of the test samples was 15 cm 14 cm (the same as the headspace measurement), given a loading ratio of m 2 /m 3. Clean conditioned air was purged through a device at a flow rate of 0.9 L/min and an air exchange rate of 1 h 1 to control the relative humidity (RH) at 50%. The concentration profile was measured at the normal room temperature (23 ± 1) C. 2.4 Methodology Air from both tests was sampled via a Tenax tube using a pump (QC-2, Beijing Institute of Labor Protection) and analyzed by gas chromatography / mass spectrometry (GC/MS) (Agilent 6850/5975B MSD). The sample injector system was a 100-tube automatic thermal desorption (ATD) system (Markes UNITY). The ATD injector system was connected by a transfer line maintained at 140 C to the GC/MSD system. The GC carrier gas was helium and the temperature program was initially set to 40 C for 4 min, and then increased at a rate of 10 C/min to 230 C and was kept constant for 5 min. The injection temperature was 230 C, the ion source type was quadrupole, and the temperature of the detector was 230 C. The cold trap was set to 10 C, and desorption temperature of the cold trap desorber was 300 C. Fig. 1 Schematic of the small-scale dynamic test-chamber system J. Chongqing Univ. Eng. Ed. [ISSN ], 2014, 13(1):

4 Identification of VOCs on the Tenax tube was performed with external standard solutions based on retention times. Mixed standard VOC solutions (benzene, toluene, ethylbenzene, p-xylene, and o- xylene, hereafter BTEX ) for identification and quantification of VOCs were obtained from the Institute for Reference Materials, Chinese Ministry of Environmental Protection. Each standard exceeded 96% purity and original solution was used at the three concentrations of 10 μg/ml, 100 μg/ml and μg/ml; then 2 μl, 5 μl, and 8 μl from the first two original solutions, respectively, and 2 μl and 5 μl from the third original solution, totally eight points were included in the calibration. The concentrations of other VOCs were calculated as toluene equivalent. Good correlations were found, with a correlation coefficient R 2 larger than for all quantified VOCs. TVOC concentration was also determined for each air sample. Sum of all detected peaks from GC with a retention time between hexane and hexadecane was estimated and then the TVOC concentration was calculated using the calibration response for toluene. 3 Results 3.1 Comparison between headspace test and dynamic chamber test The headspace test identified approximately 30 individual VOCs emitted by the three carpets. The emission composition was different for each type of carpet, as listed in Table 1. For these three carpets, the dominant compounds in the headspace test were mainly benzene and 2-ethyl-1-hexanol. Toluene was another important VOC that was detected in all the headspace tests, with a measured concentration greater than 10 µg/m 3. In Carpet 1, isopropyl alcohol was the abundant VOC; 2-methyl-2-propanol, p-xylene, styrene, acetic acid ethenyl ester, 2-butanone, and 2-pentanone were abundant in Carpet 2; and acetic acid ethenyl ester, 2-butanone, and 2-pentanone were abundant in Carpet 3. Table 1 VOC emissions from three types of aircraft carpet as determined by headspace analysis and small chamber test Compound Carpet 1 Carpet 2 Carpet 3 HS DC HS DC HS DC 2-Methyl-1-propene + Butane + 2-Methyl- butane + + Pentane + + Isopropyl alcohol ++ Acetone Methyl-2-propanol ++ + Acetic acid methyl ester Propanol + + Acetic acid ethenyl ester Butanal Butanone Ethyl acetate Trichloromethane J. Chongqing Univ. Eng. Ed. [ISSN ], 2014, 13(1): 1-10

5 Table 1 continued Compound Carpet 1 Carpet 2 Carpet 3 HS DC HS DC HS DC 2-Methyl-1-propanol ,2-Dichloro-ethane Butanol Benzene Pentanone Pentanal + 2,4,4-Trimethyl-1-pentene, + Methyl isobutyl ketone + Toluene Hexanal ++ Acetic acid butyl ester Ethylbenzene p-xylene Nonane Styrene o-xylene S-alpha-Pinene Ethyl-2-methyl-benzene + + 1,2,3-Trimethyl-benzene + Decane ,2,4-Trimethyl-benzene Ethyl-1-hexanol Undecane Methyl-2-(1-methylethyl) benzene + + Naphthalene Tridecane Mmethyl-naphthalene, Methyl-naphthalene, Total numbers Notes: HS is headspace analysis; and DC is dynamic chamber test. For headspace analysis: + refers to a concentration of 1 µg/m 3 or greater; ++, 10 µg/m 3 or greater; and +++, 100 µg/m 3 or greater (dominant compound). For dynamic chamber test (after 24 h): + refers to a concentration of 0.5 µg/m 3 or greater; ++, 5 µg/m 3 or greater; and +++, 10 µg/m 3 or greater (dominant compound). J. Chongqing Univ. Eng. Ed. [ISSN ], 2014, 13(1):

6 The experimental results of the dynamic chamber tests (see Table 1) showed that the dominant compound in the chamber tests was 2-ethyl-1-hexanol (> 10 µg/m 3 after 24 h emission). Other abundant compounds emitted by the carpets were naphthalene and 1-methylnaphthalene (> 5 µg/m 3 after 24 h). Comparison of the two test methods showed that many of the individual VOCs detected in the headspace test were not identified in the dynamic chamber test. There were only about 10 individual VOCs found in the chamber tests after 24 h emission, and the concentrations of these compounds in the chamber were very different to the headspace results. The discrepancy in the VOCs detected by these two methods is attributed to two factors. One is the carpet s strong sorption capability: Some compounds might have been adsorbed by the carpet during the manufacturing or transportation process. These compounds could be detected by the headspace test. However, the concentrations of these compounds in the carpet were very low, and therefore not detected in the dynamic chamber test. The headspace test was carried out in a static bag without ventilation. Therefore, for some easily volatized compounds, although the concentration within the carpets was low, the measured concentrations could still be very high. However, in the dynamic chamber tests, the controlled ventilation was 1 h 1, and so most of these easily volatized compounds would reduce to a concentration that was very low or below detectable limits. On the other hand, the headspace method identified those compounds that are most volatile. The compounds with a high equilibrium vapor pressure could easily volatilize from the carpets and reach a high concentration in a sealed bag. Similar experimental results were reported by Hodgson et al. [9]. Table 2 gives the physical properties of some VOCs emitted by the carpet samples. The vapor pressure of a VOC with a larger molecular mass is lower. The VOCs with a smaller molecular mass (e.g., 2-butanone, benzene, and toluene) have a higher vapor pressure, and can be readily volatilized from the carpets. Therefore, higher concentrations of these compounds accumulate in the headspace samples than those detected using the chamber method. On the contrary, in dynamic chamber tests, the initial concentrations of these compounds within the carpets were low, and they were quickly reduced by ventilation, resulting in far lower measured concentrations than those obtained by the headspace test. For the compounds with a low vapor pressure such as naphthalene and 1-methyl naphthalene, they cannot easily volatilize from the carpet; therefore, the concentrations in the chamber were at the same level compared to the those in the headspace test. The only exception was 2-ethyl-1- hexanol, which was a dominant VOC in both the headspace measurement and also the chamber test. This was due to its high initial concentration within the carpets, so it was the true dominant compound. Table 2 The molecular mass (M), boiling point (BP), and vapor pressure at 23 C (VP) of individual volatile organic compounds (VOCs) emitted by carpets VOC M BP/ C VP/kPa Butanal Butanone Benzene Toluene Ethylbenzene p-xylene o-xylene Ethyl-1-hexanol Naphthalene Tridecane These results indicate that although headspace analysis is a convenient means to screen the types of 6 J. Chongqing Univ. Eng. Ed. [ISSN ], 2014, 13(1): 1-10

7 compounds emitted from a material, the resulting measurements might give inaccurate indication of actual VOC emissions. In cases where easily volatized compounds were detected at high concentrations, we could not confirm these as the dominant compounds emitted by this material, especially for highly sorptive materials such as carpets. Dynamic chamber tests over a specified period are needed to detect the true emission characteristics. 3.2 VOC and TVOC emissions from aircraft carpets Table 3 shows the concentrations of selected VOCs in the dynamic chamber test after 24 h, and also those emitted by each carpet. From the data, it can be seen that the most important VOC emitted by the carpets is 2-ethyl-1-hexanol (concentration larger than 10 µg/m 3 after 24 h). This compound commonly originates from carpet adhesive. It has a specific taste (clearly smelled during the dynamic tests), is of mid-level toxicity; and it may cause sensitization or allergic skin reaction in some individuals [10]. Among the three types of carpet, the highest concentration of 2-ethyl-1-hexanol was emitted by Carpet 3. For other important compounds, Carpet 1 emitted the highest concentrations of ethylbenzene, xylene and tridecane; Carpet 2 emitted the highest concentrations of naphthalene, 1-methyl naphthalene and 2-methyl naphthalene; and Carpet 3 emitted the highest concentrations of benzene, toluene and styrene. TVOC concentrations in the dynamic chamber tests are also shown in Table 3. After 24 h, the highest concentration of TVOC was emitted by Carpet 2. TVOC emitted by the different carpets are given in Fig. 2. TVOC concentrations peaked within some hours after the start of the experiment and then decreased. In almost all cases this happened within the first 6 h, and was followed by rapid decay. The peak TVOC concentrations for Carpets 1 to 3 were µg/m 3 at 0.25 h, µg/m 3 at 1.25 h, and µg/m 3 at 6.75 h, respectively. Table 3 Concentrations of selected volatile organic compounds (VOCs) from each carpet after 24 h dynamic emissions Concentration/(µg m 3 ) VOC Carpet 1 Carpet 2 Carpet 3 Benzene Toluene Ethylbenzene Xylene * Styrene Naphthalene Ethyl-1-hexanol Tridecane Methyl-naphthalene Methyl-naphthalene Total * The concentration of xylene equals the sum of that of p-xylene and of o- xylene. Fig. 2 Total volatile organic compound emission from Carpets 1 to 3 over time J. Chongqing Univ. Eng. Ed. [ISSN ], 2014, 13(1):

8 3.3 Emission factors for individual VOC and TVOC from aircraft carpets The emission of TVOCs (also for individual VOCs) is often reported in terms of emission factors, calculated in accordance with ANSI/BIFMA M7.1 [11] as: E C( t ) Q i s, (1) A where A is the area of the specimen in terms; E is the emission factor; C is the concentration at time t i ; t i is the elapsed time from the start of the emission test; Q s is the chamber inlet flow rate. The approximation in Eq. (1) assumes a quasisteady-state for chamber concentration. In other words, the change of concentration in the chamber is so slow that a steady-state condition can be applied in calculating the emission factor. From Eq. (1), the maximum emission factors of selected VOCs are given in Table 4. If the concentration curves have a peak value, then the emission factors are calculated using this maximum concentration. If there is no peak value, then we use the 24 h emission concentration to calculate the maximum emission factor. The results show that Carpet 3 had the highest 24 h emission factor for 2-ethyl-1-hexanol and TVOC. 3.4 TVOC emission parameter D, K, C 0 from aircraft carpets The governing equation describing the onedimensional transient diffusion of VOCs within a material is given by Eq. (2) [12-13] : Cm ( x, t) 2 D Cm ( x,t), t where, C m (x, t) is the transient concentration of VOCs in the carpet at time t, and the diffusion coefficient D is assumed to be constant and independent of the (2) concentration. The initial condition for Eq. (2) is as follows. C ( x, t) C, t 0, 0 x L, (3) m 0 where C 0 is the initial concentration in the materials, which is assumed to be uniform throughout the material thickness; and L is the thickness of the material sample. At the material-air interface (x=l), equilibrium is assumed to be established between the material side and air side: C m (x=l, t)=k C s (t), (4) where K is the partition coefficient, C s (t) is the air-side concentration in equilibrium with the material surface. It should be noted that Eq. (2) is a simplified form of the mass transfer process in a solid homogeneous material. Table 4 Maximum emission factor E for selected volatile organic compounds (VOCs) emitted from carpets after 24 h E/(µg m 2 h 1 ) VOC Carpet 1 Carpet 2 Carpet 3 Benzene Toluene Ethylbenzene Xylene * Styrene Naphthalene Ethyl-1-hexanol Tridecane Methyl-naphthalene Methyl-naphthalene Total * The emission factor of xylene equals the sum of that of p-xylene and of o-xylene. 8 J. Chongqing Univ. Eng. Ed. [ISSN ], 2014, 13(1): 1-10

9 Key emission parameters, namely the diffusion coefficient D, partition coefficient K, and initial concentration C 0 were obtained by matching the simulated VOC concentrations with the measurement data [14]. Table 5 presents the results for TVOC, and Figs. 3 to 5 indicate that the simulated concentrations match the measured data. Notice that TVOC is not a single compound. The parameters obtained here were all equivalent values. the experiment and subsequently experienced a rapid decay. The emission parameters of TVOC emitted by all three carpets were calculated and the simulated data matched the measured data well. Table 5 Diffusion coefficient D, partition coefficient K, and initial concentration C 0 of total volatile organic compound emission from carpets Source D/(m 2 s 1 ) K C 0 /(μg m 3 ) Carpet E E7 Carpet E E6 Carpet E E6 4 Discussion We measured the TVOC and individual VOCs emitted from three types of carpets using both headspace analysis and dynamic chamber test. The results show that the headspace samples contained the largest number of volatile compounds, but many of them were not identified in the dynamic chamber tests. This is due to the strong sorption capability of the carpet and the different emission characteristics of the compounds. This disparity between the test methods means we are unable to confirm the dominant emission compounds based solely on headspace analysis, especially for highly sorptive materials, as the results might be unreliable. Therefore, dynamic chamber tests are required to identify the true emission characteristics. The dynamic chamber tests indicate that 2-ethyl-1- hexanol was a main VOC emitted from all three aircraft carpets. For all carpets, the TVOC concentrations peaked in some hours after the start of Fig. 3 Comparison between the simulated and the measured total volatile organic compound concentrations for Carpet 1 Fig. 4 Comparison between the simulated and the measured total volatile organic compound concentrations for Carpet 2 J. Chongqing Univ. Eng. Ed. [ISSN ], 2014, 13(1):

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